If the PCs get the opportunity to take a hallucinogen or are put in that state magically, go on a spirit journey, or the like, I will often ask them: what does your character see? Or, alternatively, I will sketch out a scene ("You see yourself on the back of a flying whale" or whatever) and ask them to add some details ("Where does the whale go?").
They will without fail come up with something that you:
a) Would never have thought of yourself;
b) Can incorporate into the game somehow, turning the hallucination into a "premonition" of some kind.
You have to do this sparingly, and in a subtle, non-obvious way. It's no good being literal about things - you need to get dream-logical. The best approach is opportunistic: you might never use the "whale flying to the stars" motif even metaphorically, but you never know when something star or deep-sea related will come up, and when it does...
I have been reading Arrian's Anabasis lately. It is fascinating how much ancient people relied on what they thought were the correct interpretations of goings-on in the natural world. For example:
[W]hen Alexander was still besieging Halicarnassus and was taking a midday rest, a swallow had flitted about over his head chirping loudly and settled here and there on his bed giving voice in a more than usually insistent way. Alexander was too exhausted to wake up, but the sound bothered him in his sleep and he brushed the swallow away with a light sweep of his hand. Far from flying off at his touch, the bird perched right on Alexander's head and kept going until he was completely awake. Alexander took this business of the swallow seriously and recounted it to Aristander of Telmissus, a seer. Aristander told him that it signified a plot by one of his friends, and meant also that the plot would come to light, as the swallow is a domestic bird, friendly to man, and the most talkative of all birds.
I love this kind of thing, because it is so immersive in a strange and beautiful and foreign way of conceiving of the world. How to use it in a game is difficult, but I wonder whether one method might be something like:
-There is a table of random natural events that the DM consults very occasionally
-When he does so, an event like the sparrow/nap incident takes place
-The player gives an interpretation of what has happened and what it means
-The DM writes down an interpretation of what has happened and what it means (keeping it secret from the players)
-A coin is tossed to determine which is the correct interpretation BUT the DM does not reveal the result
-Play continues: it could be that the player's interpretation of the swallow/nap incident is correct, or it could be the DM's; the players might act on the assumption that they are right, or wrong...and sooner or later they'll find out
You could. of course, do something similar with the interpretation of dreams: every so often you could roll on a Random Dream Table and ask for the players' interpretations accordingly, following a similar pattern to the above.